1. Field of the Invention
The present invention relates to a combustion state detection device, for an internal combustion engine, that detects by ion current combustion/misfire in the internal combustion engine.
2. Description of the Related Art
In operating an engine that is mostly used for automotive power, unburned fuel sometimes reaches a three-way catalyst on the occurrence of misfire. The three-way catalyst is disposed in the exhaust path so as to reduce pollutants in engine exhaust; in the event of misfire, efficiency of reducing pollutants in engine exhaust is seriously deteriorated due to deterioration of catalyst efficiency and damage of the catalyst and the like caused by the unburned fuel burning in the catalyst. Therefore, detection of misfire draws attention from the viewpoint of maintaining the environment which has raised concerns in recent years, so that its provision has begun to be mandatory worldwide.
In order to cope with this matter, a misfire detection device using ion current has been put into practical use, in which a bias voltage is applied to the center electrode of a spark plug, so as to pick up as an ion current ions produced associated with combustion, thereby discriminating between combustion and misfire in an internal combustion engine.
FIG. 5 shows an example of conventional misfire detection devices using ion current, which can be roughly split into two blocks, that is, an ignition block 501 and an ion detection block 502. The ignition block 501 includes a primary winding 501a, a secondary winding 501b and a switching element 501c; one end of the primary winding 501a is connected to the positive terminal of a battery, and the other end, to ground via the switching element 501c for interrupting a primary current. Moreover, a spark plug 503 and the ion detection block 502 are connected to the high and low voltage sides of the secondary winding 501b, respectively.
The ion detection block 502 includes a bias circuit 502a that applies a positive bias voltage to the spark plug 503, a bottom-hold circuit 502b that generates a bottom-hold threshold value for the ion current, and a waveform shaping circuit 502c that shapes a waveform by comparing the ion current with the bottom-hold threshold value so as to output a combustion pulse.
The operation of detecting the ion current will be explained next.
When a HIGH ignition signal is applied to the switching element 501c of the ignition block 501, the primary current begins to flow through a path including the primary winding 501a. If the primary current, when reaching a sufficiently high level, is interrupted by switching the ignition signal into the LOW state, a negative high voltage is generated at the end of the secondary winding 501b to which the spark plug 503 is connected. This high voltage in turn causes a spark between the center electrode of the spark plug 503 and ground.
A spark current caused by the spark flows via the secondary winding 501b from the center electrode of the spark plug 503 into the bias circuit 502a of the ion detection block 502, which charges a capacitor in the bias circuit. The capacitor continues to be charged until the voltage across the capacitor reaches the breakdown voltage of a Zener diode connected in parallel therewith, and when the voltage exceeds the breakdown voltage, a secondary current (the spark current) flows to ground through the Zener diode and a diode connected thereto in the forward direction.
When the spark current caused by the spark stops, the voltage generated by charging the capacitor begins to be applied to the center electrode of the spark plug 503 as a bias voltage for detecting ions. Moreover, the spark charges the capacitor to generate the bias voltage as well as ignites combustible air-fuel mixture at the same time. Ignition starts combustion reaction, thereby producing ions. If the bias voltage is applied to the center electrode of the spark plug 503 at this moment, an ion current flows through the secondary winding 501b from the capacitor of the bias circuit 502a toward the center electrode of the spark plug 503, which is then detected.
Following the above, the operation of discriminating between combustion and misfire using the ion current will be explained next.
To put it simply, if the ion current flows, a determination of combustion can be made, whereas no ion current flows, a determination of misfire can be done. However, depending on engine operating conditions, carbon adheres and accumulates in a space between the center and ground electrodes of the spark plug due to incomplete combustion of air-fuel mixture, so that the isolation resistance between the center and ground electrodes sometimes decreases, which in turn causes a leak current to flow along the ion current path as shown in FIG. 6C1 even if no combustion is taking place.
Since flowing of the leak current consumes electric charge that has been stored in the capacitor of the bias circuit 502a, the more the leak current flows, the more the bias voltage, which is the voltage across the capacitor, drops as shown in FIG. 6B. If the isolation resistance is assumed not to vary due to carbon during one combustion stroke, according to Ohm's low, the leak current gradually decreases demonstrating a monotonically-decreasing trend.
Under conditions of this leak current arising, if further combustion takes place and anion current arises, a current flowing along the path demonstrates a shape as shown in FIG. 6C2 in which the ion current is added to the leak current.
In the device shown in Japanese Patent Publication No. 3523542 (hereinafter referred to as Patent Document 1), a bottom-hold threshold value is used as a comparison threshold value for detecting the ion current superimposed on the monotonically-decreasing leak current. The bottom-hold threshold value, as shown by the dotted-dashed lines in FIG. 6, is such that given as an initial value a current value at time 601 that is the starting point for detecting the ion current, the threshold value is compared with a current value along a current waveform; if the current value is smaller, the threshold value is decreased toward the current value, but if not, the threshold value is kept unchanged. By comparing the current value with the bottom-hold threshold value created as above, an ion current portion 602 superimposed on a monotonically-decreasing leak current is made detectable in an appropriate manner.
However, in a case as shown in FIG. 6C3, for example, in which the starting point of the bottom-hold threshold value comes behind the point where the ion current peaks, a problem arises in that an ion current portion 603 cannot be detected.
Moreover, since flowing of a leak current decreases the bias voltage as described above, in a combustion state under the same operating conditions, an ion current volume to be detected in the presence of the leak current becomes smaller than that detected in the absence thereof, in particular, under conditions such as a low rotation speed and a low load in which the ion current level becomes low, a bottom value in some cases cannot be determined due to a reduction in the peak level of the ion current.